X-ray timer



rim-m ,0 9, X-RAY Tnvmn Donald E. Graves, Wauwatosa,.Wis., assignortoGeneral 7 ElectricCompany, a corporation .of- New York Filed June 3,1966, Ser. No.. 33,6?6 16 Claims. (Ci. 25il--95) Thisinventionrelates-to electronic means for measuring the duration of an X-rayexposure interval. More particularly, the invention concerns an X-raytimer that enables selection of intervals ranging from as low astheduration of a s'ingle'half cycle-of the'line'frequency to muchlongerv contactor in the transformer circuit and terminated by openingthe contactor. When the duration of each consecutive energizedintervalcoincides with an even number of half waves of the linefrequency,'the net magnetic flux through the transformer core is :at aminimum and there is little residual flux. On the other hand, when theinterval constitutes an odd number of half cycles or pulses, the coremay remain in a strong magnetically polarized condition betweenexposures. Then if the next train of pulses is randomly initiated, thefirst one may have set up an additive or substractive flux resulting inthe flow of normal or greater than normal X-ray tube current and,

accordingly, the quality of different films of the same radiographicsubject Would vary. This is more noticeable and less tolerable for shortinterval exposuresand for high impedance lines where regulation isgreater.

It is the general object of this invention to provide means for assuringuniform radiographic results for exposures consisting of any number ofeven or odd pulses and for insuring that the first pulse in any ensuingseries will impress on the transformer a voltage of polarity opposite tothat of the last pulse in a preceding series.

A further object is the provision of novel means for initiating eachexposure interval in a predetermined phase relationship with respect tothe line frequency.

Another object is the provision of an X-ray timer that has one mode ofoperation for time intervals corresponding with an even number of halfcycles or pulses and another mode for intervals measured by an oddnumber of half cycles.

Yet another object is the provision of means for alternating thepolarity of the X-ray tube transformer input voltage for exposuresconsisting in widely spaced single pulses between which severalnon-conductive half cycles or pulses occur. This object beingsignificant when taking a rapid sequence of radiographs on individualfilm strips where exposure intervals persist for a duration no greaterthan the time of a single half cycle of the line frequency, forinstance.

. Another object resides in energizing the X-raytransformer withalternating current from a pair of push-pull connected thyratron tubesand de-energizing the transformer at the end of an exposure interval bya timing circuit that measures the interval without regard for any phaserelationship between its energizing circuit and the pulses beingdelivered to the transformer. More specifically, this object involvesthe firing of an individual interval terminating thyratron under theinfluence of a direct current plate voltage. in another aspect, thepres- V ent'in'vention embraces interval terminating thyratrons eatstent arranged in push-pull for alternativelyenergizing an X- ray tubeaccording to which thyratron has a positive plate voltage so that allintervals for any like selector control setting are consistent.

A more specific object is to utilize a pulsed transfer relay forcontrolling theaction of a biasing rectifier and its associated relayswhereby only a single firing pulse is-delivered tothe power thyratronfor a /2 cycle exposure and .exactly an odd number of pulses isdelivered for exposures consisting in a corresponding oddnumber of halfcycles.

In general terms, the invention is characterized by a pair of push-pullconnected 'th'yratron tubes adapted to energizevan ,X-raytransformercontactor for a selected time interval consisting insany whole number ofhalf with respect to the last wavein the preceding one of a 25.

seriesof odd pulses. .When an exposure interval consisting in an even.number of. pulses is selected, the pulse relay is inactivated since .noappreciable residual flux then exists in. the X-ray transformer core andit is unnecessary to reverse. polarity on the X-ray transformer primary.

The timer also features. an'inte'rv-al terminating thyratron tube,adapted .to become conductive under the in- .,fluence of direct currentplate voltage and appropriate diminution of itsconstant direct currentbias to allow di- 35 rect currentoperation of .the time intervalterminating relay and to make termination independent of the phase angleat starting time.

Aimore specific description of what is now believed to be a preferredmode of practicing the invention'will now be set forth in reference tothe drawing in which:

FIG. 1 is a schematic diagram of an X-ray interval timer embodying theinvention; and,

H6. 2 is a schematic representation of an X-ray tube and its powersupply which is controlled by the apparatus shown in FIG. 1.

. In FIG. 2 is shown a conventional X-ray tube 10 having the usualheated filament Hand to whose anode12 is applied a fullwave rectifiedaccelerating voltage through a bridge rectifier 13. The rectifier issupplied from. a step-up. transformer 14 having a voltage selectingautotransformer 15 inits primary circuit. .Energization of the X-raytube is controlled by a contactor 16 whose closing initiates aradiographic exposure and'whose opening terminates the exposureinterval. The arrangement thus far' described may project XI-radiationonto a body 17, for instance, creating an X-ray shadowgraph on a film 18or other X-ray image storage sensitive element. The present inventionhas special advantages where film' strips 18 are to be subjected toshort exposures and ejected for allowing another strip. totake itsposition in the beam- This procedure may be utilized when performingcardiographic technic, for instance, or at any time that the radiologistwants to record rapidly changing conditions'in the organs undersurveillance. Means for transporting the consecutive film strips aresymbolized by a pair of connected cams 19 driven by a motor 24 The filmtransport mechanism may be mechani ally coupled to a timer controlswitch 21, upper left in HG. 1, the connection being symbolized by abroken line 22-22. a

The novel interval timer'will now be described in connection with FIG.l'where X-ray contactor 16 is embodied in the timer in the lower leftregion and constitutes the whose primary is fed from alternating currentpower lines L and L when line switch 29 is closed. When the circuit isinactive, but in readiness to make X-ray exposures, thyratrons 2.5 and26 are maintained non-conductive by application of a strong negativebias on their screen girds with respect to their cathodes. This bias isobtained from full wave rectifier 3%) whose output is filtered by acapacitor 31.

Grid bias resistors 32, 32' connected in parallel across the controlgrids of each thyratron 25, 26 have no voltage across them except whenexposure interval has been started by means to be described. During anX-ray exposure, consecutive positive going pulses are fed through a biasovercoming resistor 33 so that its upper end becomes more positive thanits lower. This causes a corresponding positive voltage to appear on thecontrol grids of each thyratron and 26, in which case the one having apositive plate at the time will conduct for a half cycle and terminate.On the next half cycle the adjacent tube conducts and terminates ifcurrent pulses continue to traverse the resistor 33. The voltage pulsesappearing across resistor 33 raise the control grid potentialwillciently positive to overcome the negative bias due to rectifier onthe screengrids, but these pulses are in phase with the plate voltage ofthe conducting tube, so as each plate voltage passes through zero thenegative screen grid takes over and renders that tube non-conducting.

' In the center leg 23 of the push-pull power supply there is aseries-parallel arrangement, identified generally by the referencenumeral 34, of capacitors, contacts and resistors. By controllingcertain of the contacts in a manner not essential to understanding thepresent invention, the pull-in and drop-out time of X-ray contactor 16may be modified. This is more fully discussed in the co-pendingapplication of the instant inventor, Ser. No. 708,442, filed January 13,1958, now Patent No. 2,958,782, dated November l, 196( over which thisinvention is an improvernent.

For present purposes it is sufiicient to observe that series-parallelarrangement 34 includes a normally closed safety contact 35-C in serieswith X-ray contactor 16. Safety contact 35-C is adapted to be opened bya safety relay operating coil Re 35 in the event the regular exposuretimer, yet to be discussed, fails to terminate or delays slightly beyonda preset time. The operating circuit for safety relay Re 35 will also bediscussed in more detail .later.

. In series with safety contact 35-(3 is a normally open contact 4iiBwhich when closed by its operating coil Re 49 completes the circuitthrough X-ray contactor l6 and the center leg 23 of the push-pull supplythus placing the circuit in readiness for becoming conductive when anexposure interval commences through the agency of a pulse traversingbias resistor 33.

In the illustrative circuitry of FIG. 1 all exposures are started byclosing switch 21 either mechanically or by its closure and as a resultof connecting relay coil Re 4i) between L; and L This closes the circuitthrough X- ray contactor is by way or" contact sc-n as discussed earlierand prepares the circuit for timing.

The circuitry may be considered most conveniently as being divided intosections identified in FIG. 1 as follows: Start and pulse; terminate;safety; and, pulse sequence selection. Elements in the start and pulsesection will be identified concurrently with a description of their modeof operation. I

'A timed X-ray exposure interval commences when a start relay coil Re isenergized. As can be seen in the drawing, coil Re 45 is in series with anormally closed stop relay contact -A and is connected to line L Also inseries with coil Re 45 and contact Re Sil-A is a start rectifier 41which has at least one normally closed path at all times. Rectifier 41is in series with a normally open contact all-D from which a connectionis made to line L Thus, when hand switch 21 is closed, for instance,relay Re 4% is energized, by virtue of being connected between line Land L in which case contact til-D closes to complete the circuit throughexposure start relay coil Re 45 and an exposure commences.

Start rectifier 41 comprises parallel legs, one of which includes adiode rectifier 42 in series with a contact'SS-C which is shown closed.The other leg includes an oppositely polarized diode rectifier 43 and acontact -D shown open. Contacts 55-C and 55-D are controlled by atransfer or pulse relay Re 55 to be described later.

a It will be shown that the last mentioned contacts may be alternatelyopened and closed for one mode of timer operation, that is, when asingle or odd number of halfcycles constitutes the timed interval, butin the general case, when there are an even number of half cycles, onerelay contact 55-C or 55-D will remain closed.

When exposure initiating contact id-D closes, current flows from line Lserially through iii-D, SS-C, diode 42, 5i A and exposure start relaycoil 45, energizing the latter.

Energizing exposure start relay Re 45 causes anum-ber of simultaneousevents in the circuitry. The eflect on the circuit including push-pullbias rectifier 46 will first be examined. Note that alternating currentis fed to bias rectifier 46 from lines L and L by leads 47 and 48. Inlead 48 are three series connected contacts: id-F, closed by closing themanual switch 21; normally open '45-B, closed by energizing exposurestart relay Re 45; and normally closed contact 5'0C which is opened toterminate all exposures as will be described later. Thus, when the threelast mentioned contacts close, bias rectifier as has an alternatingcurrent input.

The direct current output of bias rectifier 46 is fed to the thyratronpower supply bias resistor'33 and this output may comprise a single halfWave pulse or a continuous series of positive pulses, depending upon themode of operation selected. Rectifier 46 is fundamentally of thefull-wave bridge type modified to suit the present invention.Accordingly, one junction of the rectifier is opened and has a normallyopen contact dilE in the gap. As will be more fully expalined later,69-5 is usually closed during most operations but may be opened 7 whenit is desired to operate bias rectifier 46 as a half wave rectifier asis the case when a single exposure consisting in the same duration as ahalf Wave of a line frequency is selected and when an exposureconsisting in an odd number of half cycles is underway. Also airiliatedwith bias rectifier 46 are a pair of relay contacts SS-A and 5543, theformer being shownclosed and the latter opened, connected in parallelwith each other and in series with a lead 49 feeding direct currentpulses to thyratron bias resistor 33. Contacts 55A and 55 may bealternately opened and closed by pulse relay Re 55 during one mode ofoperation to be discussed later, but in any case, one or the other isclosed to complete the direct current circuit of bias rectifier 26.

It is convenient at this point to assume, by means yet to be described,that t ough the timer an X-ray expo-sure interval consisting in an evennumber of half waves of line frequency has been selected. For instance,assume .nate positions after each exposure so that a case where' gsecond exposure or 8 half-cycles of. a

t 60 cycle'line'frequency takes place after: which bias rectifier 46would be disconnected from its A.C. inputby opening of exposure stoprelay contact Sti-C. In such case, contact 6ti-E would be closedvantl'contact 55'A may remain closed as showri,-so that 8 unidirectionalhalf cycle'pulseswould appear across bias resistor-:33. This meansthatpush-pull thyratrons 2S and 2o' would conduct alternateiy for a total of4 half cycles each and that X-ray contactorl would close for a totalofthe' words, the first half of the first half wave starting: pulse itwill wait for'ft he next half wave before' it oper'ates'to initiate anexposure. It' operates in point of, time when the voltage applied to itis just beyond its peak and when it does operate it closes contact 45:3to energize bias i rectifier 46. "This causes bias rectifier '46, andaccordingly, bias resistor 33 to be energized wlth-a consistentgrid'signal. Any delay imposed by the time'required to close contact 45-B isaccountedfor in the total interval by the termination circuit yetto: be described;

it the chosen exposure interval consists in an odd numer of half waves,such as only one half wave, means are provided forassuring-that the"first voltage half wave 1 of a series that is impressed on'thetransformer 14 will be of opposite polarity to the last in theprecedingseries. For the time being it is sufficient to observe that pulse relayRe '55 may be activated forthis purpose. and that it may cause alternateopening and; closing of its contacts 55 A and 55-13 associated'with'biasrectifier 46 to bring this about. Assume that by appropriate setting,pulse relay coil Re 55 is activated for an-odd cycle exposure at thebeginning of'which relay contact SS-A is closed. it the pulse seriesconsists in single pulses such as a dozen or so half waves in a second,contact ii-E will start open and remain open. if the series is acontinuous one totaling an odd number 6 =3E may be open at the start andclosed with termination. Current then flows from the alternating currentinput line48 through diode 51, from the DC; positive terminal ofrectifier 46, through'bias resist-or 33 to make its upper end positive,back through relay contact 55-A, through diode 52 and out at alternatingcurrent line .7. If there are, for example, 7 half wave pulses passedbefore termination of the interval by opening stop contact 5643, onethyratron,

cit er 25 or 26 would fire tour timesand the other three times and theywould be phased with the line and X-ray transformer 14 voltages. At theend of the series, contact 55-A would open and Contact 55 3 would closeby operation of pulse relay Re 55 so that bias rectifier 4-6 would haveto first see the opposite half Wave of A.C. input voltage before'itconducted in order to first fire the thyratron that fired fewer times inthe preceding series. This is controlled by the pulse relay Re 55 andstart rectifier 41 now to be described in detail. For spaced half waveor 3 second exposures, ii-E would remain open but contacts SS-A and 55-Bwould alterbias rectifier .6 would operate on half waves throughalternate branches.

When any odd or even number of pulses constitutes the exposure intervalcontactSG B in the start and pulse circuit closes at termination,-but onan odd number an automatically selected relay also closes contact 65-13at this time. It is desirable that these contacts Sit-B and 65-D remainopen during an exposure lest they affect the pull-in time ofexposurestart relay Re'45. When they close, a capacitor 56 is charged to apolarity dependent .upon which contact SS-C or SS-D allows conductionthrough its associated diode 4-2 or 43, respectively. This charge isheld on capacitor'sountil the manual or automatically driven switch, 21is opened in which case relay coil. Re 70 is de-energized and relaycon-tact fill-A, in

vseries with'pulse' relay .ReSS, closes. This permits capacitor'56 todischarge through pulse relay 55A in the bias rectifier 4 6, and toclose contact 55-1) and open contact 550 in the start rectifier 41.gThus-as described above, bias rectifier 4-6 and start rectifier'lremain synchronized because the exposure-start relay Re 45 must see anopposite polarity halfcycle to start the nextexposure because reverseddiode 43 will be the-first to conduct in the next-series. At theend-ofthe next odd series, pulse relay Re '55 will transfer back and, repeat.the alternation of the various contacts.

Pulse relay R2 55 is obtainable commercially and it maybe of thelatching type. One type, not" shown in detail, includes apermanent'magnet rotor which executes 'a'half revolution and latches inresponse .to itsfield winding'being energized with current havingonepolarity. Upon current reversal in its field, such as resultsfromoppositely directed discharge current from, pulse capacitor56'traversing its winding, the mutual forces of-magnetic' repulsion cause ahalf revolution and latching in a diametrically opposite rotor position.

Where an exposure interval consisting in a single half wave or 9 secondis selected, by means yetto be ,and the latter operates as a half waverectifier. The

described, Contact 6 ii-r-E in the bias rectifier46 is open purpose ofthis is to prevent more than one complete "pulse from being deliveredtorbias resistor 33 lest the exposure period also persist beyond theintended one pulse due to inadvertent firing of both thyratrons in rapidsuccession. During the non-conductive half cycle immediately followingthe conductive single half cycle of bias rectifier 46, and in fact,during several ensuing half cycles, time is available to allow for theinertia and operating delay of the transferable contacts discussedabove. A commercial formof the timer, nevertheless, allows as many astwelve second exposures spaced by-such inactive periods within onesecond. These inactive periods are convenient for transporting a newfilm 18 strip into the X-ray beam during rapid sequence radiography asexplained earlier in reference to FIG. 2.

It will be observed that when contact Gil-E is opened and contactSS-Aclosed as shown in FIG. 1, that if A.C. input line 43 is positive at theassociated bias rectifier 46' terminal, that current will flow throughdiodeQSl to the DC. plus terminal, again making the upper end of biasresistor 33 positive from whence current returns to the rectifierthrough contact 55-A, and passes through diode 52, whereupon it returnstothe then negative AC. input wire 47. On the next half cycle, when theA.C. input voltage changes polarity on the rectifier, contact 55-A maystill be closed and Contact 55-B openedso that the return path isblocked by the reverse impedance of the other pair of diodes and becausecontact 60-E s open. Thus, any delay in the action of theiterminatmg orexposure measuring circuit. during, half cycle exposures will not causethe interval to be extended and conductive at alltimes except at the endof a measured interval, a constant negative bias of approximately,

volts is applied to its control grid with respect to line L This isachieved by a voltage divider bridge 73 that has a unidirectionalvoltage impressed on it by connecting it at one end to a series dioderectifier 74-, attached to line L and at its other end bridge 73 isattached to line L A smoothing capacitor 75 is also connected across thevoltage divider 73. An adjustable tap 76 in series with a contact 45Cimpresses the proper negative bias on the control grid of tube 72 and italso allows timing capacitor 77 to charge to the same negative potentialon its side which connects to control grid of tube 72 through a limitingresistor. When an exposure is started by energizing relay Re 45 asdescribed earlier, Contact '45-C opens, thus removing the constantnegative bias on tube '72 and simultaneously a relay contact 45-Dcloses. This connects capacitor 77 for discharging through a part of aseries of time selecting resistors collectively designated 79, through amovable tap 80, and a gas-filled regulator 81 which also connects toline L through a limiting resistor 82. The time of discharge, andaccordingly, the time interval elapsing before thyratron tube 72conducts and stops the exposure depends upon the selected position ofpotentiometer tap 80 on the series resistance group 79.

Connected to the relay Re 59 and hence indirectly to the plate of tube72 is a storage capacitor 83 whose one side is negatively charged andconnected to line L,

and whose other plate is positively charged by reason of being connectedto line L through a diode rectifier V 84 and resistor 82. The positivepotential on capacitor 83 appears on the tube 72 plate with respect toits cathode and is sufficient to fire the tube after timing capacitor 77discharges to the critical firing voltage of the tube. Capacitor 83 islarge enough to hold the plate of tube 72 positive during anyconceivable interval, and the tube is efiectively under the constantinfluence of a direct current plate potential during conduction despiterectifier diode 84 not conducting and not recharging capacitor 83 when Lpasses through a negative half cycle. Thus, the terminating circuit doesnot require that its timing period be commenced in any particular phaserelationship with the start of the exposure or in coincidence with thestartof the first pulse through pushpull bias resistor 33 since it timeson an absolute basis measured in terms of direct current flow ratherthan the number of pulses fed to the X-ray tube. Filter capacitor 83 hassufficient energy to hold the plate of thyratron 72 and gas-filled diode81 positive during conduction. This also causes the grid of 72 to swingsharply positive by reason of the positive voltage on the anode of gasregulator tube 81 appearing on it through timing resistor bank 79 andcontact 45-1). Hence, tube 72 is influenced by a sharply rising positivegrid to reach full current flow immediately after passing throughcritical timing capacitor 86 yield a slightly longer time constant.

The safety timer includes a thyratron tube 87 which when renderedconductive energizes a relay coil Re 35 in series with it, causingopening of a contact 35-C residing in the center leg 23 of the push-pullsupply and which is effectively in series with X-ray contactor 16.

Safety timer tube 37 is normally biased'non-conductive by application ofa negative potential to its grid, this potential being derived from atap 88 on one side of voltage divider bridge 73 through a normallyclosed contact 45-E. The negative potential also appears on safetytiming capacitor 86. When an exposure is started by energization ofrelay Re 45, contact 45E opens, isolating the grid of 87 from its biassource. Contact 45-F closes, connecting the group of safety timingresistors 89 to capacitor 86. The capacitor 86 discharges through aresistor dependent upon the position of the movable arm 90 through lineL and to the opposite side of capacitor 86. This fires tube 87 as asafety back-up.

Attention is now invited to the pulse sequence selection section, at thefar right in FIG. 1, where the timer is conditioned for its variousmodes of operation. That is, for spaced single half Wave exposures, anodd number of continuous half waves, or an even number of half waves.

This section comprises three relay operating coils Re 6%, Re and Re 65each of which has in series circuit an associated deck of contacts 96,97 and 98, respectively, constituting selector switches. Each deck hasmovable switch arms 96', 97' and 98, respectively, that are coupled forjoint operation with each other and with the selector arms 8% and 90 onthe terminating and safety resistor groups 7 and 89.

Relay coil Re 60 is adapted to be energized for all exposures consistingin an odd or even number of half waves, except single half waves or 1second exposures if time is reckoned on the basis of a sixty cyclesupply.

Relay coil Re 95 is adapted to be energized during any exposureconsisting in an even number of half waves duration.

Relay coil Re 65 is adapted to be energized during exposuresconstituting any odd number of continuous half waves including single,intermittent half waves.

An exposure consisting in one half waves of one polarity followed by anoff-time of several half waves and then another half wave of oppositepolarity, and so on, will be considered first. For instance, a series ofA exposures spaced from each other or just one such pulse will beconsidered. In this case, contact 60-E in the bias rectifier 46 shouldopen and remain open so that passing of consecutive pulses throughopposite branches of the rectifier is possible. Thus, for singlealternate pulses tap 96 rests on a dead terminal in deck 96 so that Re66 remains de-energized. Bias rectifier 46 then operates as a half waverectifier so' that the alternate half waves 11: passes are phased withcorresponding half waves supplied to X-ray transformer 14 in order toavoid magnetically saturating it. During this mode of operation it isalso necessary to activate pulse relay Re 55 so it causes transfer ofits contacts 55-A and 55-8 associated with the bias rectifier 46, andcontacts 55-C and 55-D associated with the start rectifier 41 asexplained earlier. The total number of exposures may be controlled bysequencing switch 21 manually or automatically with film ejector cam21'.

To condition Re 55 for pulsing, Re 65 coil is energized by connectingmovable arm 98 to the topmost contact, for example, in deck 98 tocomplete a circuit through the coil between line L and L This closescontact 65-D in the start-and-pulse section and enables pulse capacitor56 to charge after each single half wave exposure and to dischargethrough the pulse relay Re 55 by reason of contact 70A closing when themanual or automatic switch 21 opens to deenergize coil Re 70.

When the pulse relay Re 55 operates, it may open contact 55A and close55-B so that bias rectifier 46 must wait until an opposite half of theA.C. input wave appears before it can conduit and deliver another biaspulse to bias resistor 33. Consequently, the thyratrons 25 and 26 willalso fire alternately and in phase with the A.C. half wave supplied tobias rectifier 46 and the X-ray transformer 14.

On consecutive exposures, contacts SS-C and 55-D in the start rectifier41 will also transfer, causing pulse capacitor se to charge oppositelyand pulse relay Re 55 to take opposite positions. Transfer of this pairof contacts 55-C and 554) also assures that exposure start relay Re 45will pick-up on an oppositely polarity half cycle when the nextconsecutive exposure takes place.

Recapitulating, exposure start relay Re 45, pulse relay Re 55, biasrectifier 46 and the X-ray transformer 14 all see the same consecutivedifferent half wave voltages at the same time when a series of half wavepulses spaced from each other constitute consecutive exposure intervals.The type of exposure interval now to be examined is one where a seriesof continuous half waves, totaling an odd number, measures the interval.In this case the line voltage may go through one additional positive ornegative half wave. An example would be a 4 Second exposure consistingin three half cycles at sixty cycle line frequency where the taps 8i and9! of the coupled time selectors are moved to positions on theirresistor banks 79 and 89 for yielding a time constant of that durationin conjunction with their timing capacitors 77 and '86,

respectively. In this case selector arm 98' would be shifted to acorresponding live terminal in deck 98' and relay Re 65 would energize.'Ihis closes contact 65-D and prepares pulse capacitor 56 for beingcharged and operating pulse relay Re 55 at an appropriate time as isnecessary for any odd pulse sequence.

By positioning tap 98, tap 96 in deck 96 will also be compelled to takea position on an appropriate line terminal so that relay coil Re 69 willenergize. Relay Re would not be energized because of tap 97' resting ona dead terminal.

With Re 66 energized, contact 6943 in bias rectifier 46 closes when theodd number of pulses time selection is made, thereby setting up biasrectifier 46 to operate full wave. When exposure start relay Re 5 picksup, a thyra-tron 25, for instance, will fire upon receiving a full halfwave biasing pulse from rectifier 46. The polarity of the first halfcycle passing from the fired thyratron to the X-ray transformer 14 willhave been predetermined by the rest positions of contacts 55C and 55-D,in start rectifier 41, which decide which half cycle of line voltagewill bring in start relay coil Re 45. If 55-C were closed at thebeginning it will continue to feed through diode 42 to operate Re 45 onpulsating direct current for the duration of the odd cycle exposure.When the exposure terminates by firing tube '72 and operation of Re 5%and its associated contacts, pulse relay Re 55 will operate to switchcontacts 55C and 55-1) in start rectifier 41. Hence, the next exposurewill begin when the line polarity is opposite of that at the beginningwd end of the last exposure due to the first pulse in the second seriesbeing passed by contact 55-D and rectifier 43. The other thyratron 26,for instance, will then be the first to tire for passing a differentlypolarized voltage wave than the last in the last series to X-raytransformer 14 with the benefit of avoiding saturating its core.

The exposure is terminated by opening stop contact Sit-C and cutting offthe A.C. supply to bias rectifier 46. At this time relay contact 5d-Bcloses to connect pulse capacitor 56 for charging with a very short timeconstant. When manual and automatic switch 21 is opened, contact 7t'3-Acloses to discharge capacitor 56 through pulse relay Re 55, therebytransferring contacts SS-A, B, C and D to condition start rectifier 41and bias rectifier 46 to start conducting on an opposite half wave forthe next exposure as explained above.

An exposure constituting an even number of half waves will now beconsidered. For instance, a second'or four half waves exposure interval.Here, no problem of transformer saturation arises, of course, so .biasrectifier 46 operates full wave and pulse relay Re 55may remain dormant.

In this mode of operation arm97' in deck 97 may be connected with a liveterminal at which time decks 96 and 98 remain open. Closing the circuitthrough deck 97 energizes relay coil Re which in turn closes its contact95-A so that relay coil Re 6t) is connected between lines L and L also.Energizing coil Re 60 causes its contact ell-E in the bias rectifier 46to close, thereby connecting the-latter for full wave operation. Whenthe timing interval starts after energizing start relay Re 45, biasrectifier 46 delivers a series of positive pulses to bias resistor 33and thyratrons 25 and 26 continue to fire alternately according to whichhas a positive plate, until the exposure is terminated.

In summary, an X-ray exposure interval timer has been described thatstarts in response to the alternating current half wave it sees andterminates after a precise interval measured in terms of direct ctu'rentflowing in an RC timing circuit. The invention assures that intervalscomprising odd numbers of half waves are initiated on consecutivelyopposite polarities of line voltage for avoiding saturation of the X-raytransformer core. It allows as short as one second exposure or a seriesof them within one second. A' commercial form permits exposures up totwelve seconds duration with equal precision.

Although an embodiment of the invention has been described inconsiderable detail this is not to be interpreted as limiting, for theinvention may be variously embodied by those versed in the art and is tobe construed according to the scope of the claims'which follow.

' It is claimed:

L An X-ray' exposure interval timer comprising an X-ray'tube supplytransformer; switch means adapted to control energization of saidtransformer, a push-pull power supply including a pair of electron tubesin circuit with said switch means; means-normally biasing said tubesnon-conductive and a bias resistor across which a voltage may bedeveloped to overcome said bias, rectifiermeans delivering aunidirectional output current that traverses said bias resistor inresponse to an alternating input voltage, start relay means adapted toclose a circuit that impresses stud input voltage, power lines providingacommon source of alternating voltage to said start relay means and saidrectifier means,

means adapted to connect said start relay means to said power lines whensaid alternating voltage has a predetermined polarity at the beginningof one interval so as to apply said input voltage to said rectifier withthe same polarity, and means operative when said preceding intervalconsists in an odd number of half-cycles for connecting said start relaymeans to said power line at the start of the next interval when saidpower line has an opposite polarity.

2. The invention set forth in claim 1 wherein said means for connectingsaid start relay means comprises a parallel loop circuit in series withsaid start relay means, each leg of said loop including an oppositelypolarized rectifier and a contact in series therewith, and meanssimultaneously closing one contact while holding the other open topredetermine which polarized rectifier will conduct and energize saidstart relay means.

3. The invention set forth in'claim 1 including means controlling saidmeans for connecting said start relay means to said power lines at apredetermined polarity, said controlling means comprising an impulserelay having operated contacts in circuit with said connecting means andsaid impulse relay being adapted to change said contacts alternatelyopen and closed, respectively, in response to single voltage impulsesafter termination of an X-ray exposure interval.

4. An X-ray interval timer comprisingan X-ray tube supply transformer,switch. means adapted to control energization of said transformer andto'establish an ex- 'po sure interval, a push-pullpower supply includinga pair of electron ltuhesin circuit with said switch means,

means normally biasing said tubes non-conductive and .a biasing resistoracross which a voltage may be developed to overcome said normal bias,rectifier means delivering a unidirectional output current traversingsaid bias resistor in response to an alternating input voltage, startrelay means having a contact for closing circuit to said rectifier meansto initiate a timed exposure interval, a common alternating currentsupply for said start relay means and said rectifier means, a pair of oppositely polarized rectifiers having in series respective alternatelyopenable and closeable contacts and in parallel with each other in aseries circuit with said start relay means, an impulse relay meansoperating s-aid alternately openable and closeable contacts after anexposure interval to alternate positions whereby the start relay meanswill begin conducting at the next timed interval when the alternatingsupply voltage polarity is different than at the beginning of thepreceding interval, thereby impressing a difierent initial polarityvoltage on said transformer for consecutive intervals.

5. An X-ray interval timer comprising an X-ray tube supply transformer,switch means adapted to control energization of said transformer duringan exposure interval, a push-pull power supply including a pair ofelectron tubes in circuit with said switch means, means normally biasingsaid tubes non-conductive and a bias resistor across which a voltage maybe developed to overcome said normal bias, a first bridge rectifiermeans delivering a unidirectional current traversing said bias resistorin response to an alternating input voltage, a first contact interposedbetween a junction of said bridge rectifier means, an output line fromsm'd bridge rectifier, a first pair of alternately openable andcloseable contacts in parallel with each other and respectivelyconnected to opposite sides of said first contact in the junction andjointly to said output line, means for opening said first interposedcontact to thereby open the bridge rectifier for half-wave rectificationthrough one path thereof, an alternating power source for said bridgerectifier, means transferring said pair of contacts to alternatepositions .at the end of a timed interval whereby said bridge rectitierconducts through a different path and thereby conducts during the nextinterval when the alternating power source has a diiierent initialpolarity than at the end of the preceding interval. 7

6. "the invention set forth in claim including an impulse relay meansoperating said pair of contacts, said impulse relay means having acontact in series with it, a storage capacitor in parallel with saidimpulse relay means and said series contact, means for charging saidcapacitor at the end of a timed interval and means to close said seriescontact in sequence for discharging said capacitor through said impulserelay to cause transfer of said pair of contacts.

7. An X-ray interval timer comprising an X-ray tube supply transformer,switch means in circuit with said transformer, a power supply adapted toenergize said switch means for a timed interval, first rectifier meanscontrolling said power supply, start relay means including switchcontact means for closing a circuit to said first rectifier means, analternating current source, second rectifier means having two parallelpaths that are alternately conductive and in series with said relaymeans across said source, means for closing said paths sequentially toenergize said relay means in accordance with the polarity of the source,whereby said switch contact means connects said first rectifier meansacross said source in synchronism with said start rectifier means andconsecutive intervals start with different polarity voltages beingapplied to said transformer.

8. The invention set forth in claim 7 including an interval terminatingtimer, said timer including an electron tube having a cathode, grid andanode, means applying a biasing voltage between said grid and cathodemeans for overcoming said bias to render said tube 12. conductive, arelay switch means in series with said anode and adapted to be energizedwhen said tube conducts, and contacts controlled by said relay switchmeans for disconnecting said start relay means and said first rectifiermeans when said tube conducts at the end of an interval.

9. in an X-ray exposure interval timer, an X-ray source, a transformerfor energizing said source, an alternating current power line, switchmeans for connecting said transformer to said power line, a power supplyoperating said switch means at the beginning and end of a timedinterval, start relay means including switch contact means controllingsaid power supply, a pair of reversely polarized rectifiers each havingan alternately openable and closeable contact in series therewith toform parallel branches in series with said start relay means and acrosssaid power line, said relay means thereby initiating its conduction independence upon the polarity of said power lines and upon which of saidopenable and closeable contacts is closed, an impulse relay means and acapacitor in parallel therewith, said impulse relay means operating saidopenable and closeable contacts, means for charging said capacitor atthe end of, an interval, means for discharging said capacitor to operatesaid impulse relay means and transfer said contacts and complete adifferent circuit through said rectifier means, whereby said start relaymeans will be energized at the beginning of the next consecutiveinterval when the power line voltage has a diiierent polarity than atthe end of the preceding interval.

10. In an X-ray interval timer, an alternating current power line, anexposure start relay means adapted to start an interval when energized,a pair of reversely polarized rectifier means each having an alternatelyopenable and closeable contact in series therewith to form parallelbranches jointly in series with said start relay means, the onerectifier whose series contact is closed being conductive at the startof an interval and determining the power line polarity at which thestart relay begins conducting, an impulse relay means operabletotransfer said contacts at the end of a timed interval to close the otherrectifier circuit, whereby said start relay means will become energizedwhen said power line has an opposite polarity at the beginning of thenext successive interval than it had at the end of the last interval.

11. In an X-ray interval timer, an alternating current power line, meansfor determining the polarity of said power line at the beginning of atimed interval, said polarity determining means comprising a relay meansconnectable to said line to start a measured interval, a pair ofreversely polarized rectifin means each having a respective normallyopen and closed contact in series therewith, the closed contactrendering its associated rectifier and the starting relay meansconductive when the power line has one polarity of the beginning of oneinterval, means for transferring said contacts at the end of oneinterval to thereby close circuit through an aiternate polarizedrectifier whereby said relay means will begin conducting when said powerline polarity is difierent than at the end of the last interval.

12. An X-ray exposure interval timer adapted to meas ure intervalsconsisting of individual half waves ,of a power line frequency,comprising, a power line, an X-ray source, a power supply for saidsource, said power supply including a pair of push-pull connectedelectron tubes normally biased non-conductive and adapted to be renderedselectively conductive in accordance with positive voltage appearingthereon, a bias resistor adapted to develop a voltage in response tobeing traversed by a signal that overcomes said normal bias and rendersone of said tubes conductive, a bias rectifier of the bridge type forperiodically'energizing said bias resistor and having a controlledcontact in a junction thereof, a normally open and a normally closedcontact in parallel with each other and respectively connected toopposite sides of said controlled contact to thereby establish oneconduction path through said bias rectifier at the beginning of oneinterval, and means for transferring said contacts to establish adifferent conductive path through said bias rectifier at the end of saidinterval whereby an alternate of said tubes will conduct at thebeginning of the next interval.

13. An X-ray interval timer comprising an X-ray source, a transformerfor energizing said source, an alternating current power line, switchmeans connecting said transformer to said alternating current power lineat the start of an X-ray exposure interval, start relay means which whenconnected to said alternating current power line operates said switchmeans to start a timed interval, an electronic interval terminatingcircuit including an electron tube having an anode, cathode and controlgrid, means applying a bias voltage between said grid and cathode forrendering said tube non-conductive, means for diminishing said biasvoltage after a predetermined time to render said tube conductive, aterminating relay means in series with the anode and cathode of saidtube and having a contact for opening to de-energize said transformerwhen said tube conducts, a storage capacitor in parallel with said tubeand terminating relay for impressing a predetermined direct voltagethereon when the tube conducts, whereby the voltage applied to the tubeis independent of phase relation and magnitude of the power line voltageat the beginning and end of an interval.

14. An X-ray interval timer including means for selecting exposureintervals consisting in individual alternate polarity spaced half waves,odd numbers of half waves and even numbers of half waves of analternating power line voltage, a power line, an X-ray transformer, apower supply, means for periodically energizing said transformer fromsaid power line through said power supply, start relay means connectableto said alternating voltage power line to activate said power supplyupon initiation of an exposure interval, means for connecting said startrelay means to said power line with a predetermined polarity, aplurality of jointly operable selector switches each having a relaymeans in circuit for connection to said power line in accordance withthe selected positions of said selector switch, said start relay meanshaving in circuit therewith a pair of alternate current paths conductivein dependence upon polarity of said power line at the start of aninterval, said paths each having series contact means, a pulse circuitincluding impulse relay means and a capacitor in parallel therewith,means controlled by one of said selector switches for energizing saidcapacitor through one of said paths when individual or odd numbers ofhalf waves constitute the interval, means for connecting said impulserelay means to said capacitor to operate the contacts in said paths andfor changing their conduction polarity at the end of each interval.

15. The invention set forth in claim 14 wherein said power supplyincludes a pair of electron tubes in push-pull relationship, meansbiasing said tubes non-conductive, a bias resistor across which avoltage may be developed for overcoming said bias and firing whichevertube is positive, a bias rectifier of the bridge type having a contactinterposed in a junction thereof, a pair of rectified current paths eachhaving an open and a closed contact therein connected to opposite sidesof said interposed contact, one of said selector switches controllingsaid contact to effect two paths through the bias rectifier whereby halfwave unidirectional pulses may be delivered to the bias resistor, andsaid path contacts being transferable by said impulse relay means tochange paths in synchronism with appearance of half waves of differentpolarity on said power line.

16. In an X-ray tube exposure interval timer circuit, wherein an X-raytube may be energized during exposure intervals consisting of acontinuous odd or even number of half cycles of a power line frequencyor during spaced half cycles of alternate polarity, an A.-C. power line,an X-ray tube, a step-up transformer for energizing the X- ray tube fromthe power line, a bridge rectifier coupling the secondary winding ofsaid transformer to said X-ray tube through the same direct current pathfor any of the aforementioned exposure intervals, contactor means incircuit with the primary winding of the transformer, said contactormeans being adapted to close during any continuous interval and to openat the end thereof and to close and open for each interval consisting ofspaced half cycles of alternate polarity, start relay means for startingan interval, a parallel circuit including means selectively conductivein accordance with power line polarity, said parallel circuit being inseries with said start relay means and connected to a power line,transfer means for rendering conductive one of said parallel paths forone line polarity and the other path for another line polarity before anexposure interval, a control circuit supplying said contactor means,means for closing said control circuit under the control of saidtransfer means for initiation of conduction on a polarity correspondingwith that during which the start relay begins conducting, whereby saidcontactor means may close the circuit through the primary winding of thetransformer when the polarity of the voltage on the latter is oppositeto that at the end of the preceding half cycle ending the previousinterval.

References Cited in the file of this patent UNITED STATES PATENTS

16. IN AN X-RAY TUBE EXPOSURE INTERVAL TIMER CIRCUIT, WHEREIN AN X-RAYTUBE MAY BE ENERGIZED DURING EXPOSURE INTERVALS CONSISTING OF ACONTINUOUS ODD OR EVEN NUMBER OF HALF CYCLES OF A POWER LINE FREQUENCYOR DURING SPACED HALF CYCLES OF ALTERNATE POLARITY, AN A.-C. POWER LINE,AN X-RAY TUBE, A STEP-UP TRANSFORMER FOR ENERGIZING THE XRAY TUBE FROMTHE POWER LINE, A BRIDGE RECTIFIER COUPLING THE SECONDARY WINDING OFSAID TRANSFORMER TO SAID X-RAY TUBE THROUGH THE SAME DIRECT CURRENT PATHFOR ANY OF THE AFOREMENTIONED EXPOSURE INTERVALS, CONTACTOR MEANS INCIRCUIT WITH THE PRIMARY WINDING OF THE TRANSFORMER, SAID CONTACTORMEANS BEING ADAPTED TO CLOSE DURING ANY CONTINUOUS INTERVAL AND TO OPENAT THE END THEREOF AND TO CLOSE AND OPEN FOR EACH INTERVAL CONSISTING OFSPACED HALF CYCLES OF ALTERNATE POLARITY, START RELAY MEANS FOR STARTINGAN INTERVAL, A PARALLEL CIRCUIT INCLUDING MEANS SELECTIVELY CONDUCTIVEIN ACCORDANCE WITH POWER LINE POLARITY, SAID PARALLEL CIRCUIT BEING INSERIES WITH SAID START RELAY MEANS AND CONNECTED TO A POWER LINE,TRANSFER MEANS FOR RENDERING CONDUCTIVE ONE OF SAID PARALLEL PATHS FORONE LINE POLARITY AND THE OTHER PATH FOR ANOTHER LINE POLARITY BEFORE ANEXPOSURE INTERVAL, A CONTROL CIRCUIT SUPPLYING SAID CONTACTOR MEANS,MEANS FOR CLOSING SAID CONTROL CIRCUIT UNDER THE CONTROL OF SAIDTRANSFER MEANS FOR INITIATION OF CONDUCTION ON A POLARITY CORRESPONDINGWITH THAT DURING WHICH THE START RELAY BEGINS CONDUCTING, WHEREBY SAIDCONTACTOR MEANS MAY CLOSE THE CIRCUIT THROUGH THE PRIMARY WINDING OF THETRANSFORMER WHEN THE POLARITY OF THE VOLTAGE ON THE LATTER IS OPPOSITETO THAT AT THE END OF THE PRECEDING HALF CYCLE ENDING THE PREVIOUSINTERVAL.